1. Field of the Invention
The present invention relates to a method of preventing deterioration of a palladium oxide type anode. More particularly, it relates to an improvement of durability of a palladium oxide type anode by preventing weight loss of palladium oxide caused by actuating a jumping switch (by-pass) for stopping the operation of a diaphragm type alkali metal chloride electrolytic cell equipped with a palladium oxide type anode.
2. Description of the Prior Arts
Electrodes having a surface layer comprising a metal oxide as a platinum group metal oxide have been proposed as anodes of the alkali metal chloride electrolytic cell for producing chlorine in anode compartment and an alkali metal hydroxide in a cathode compartment by an electrolysis of an aqueous solution of an alkali metal chloride to obtain dimensional stability for long period and low overvoltage of the anode. Among them, an anode made of a valve metal substrate coated with ruthenium oxide has been practically used in view of excellent characteristics in U.S. Pat. No. 3,711,385 and No. 3,864,163.
Recently, an electrode having the surface coated with palladium oxide among the platinum group metal oxides has been proposed as an excellent anode with high oxygen overvoltage for producing chlorine having high purity in the anode compartment and low chlorine overvoltage as disclosed in Japanese Unexamined Patent Publication No. 35277/1974, No. 43879/1979 and No. 77286/1979.
On the other hand, when an electrolysis of an aqueous solution of an alkali metal chloride is carried out in a diaphragm type alkali metal chloride electrolytic cell equipped with a porous diaphragm or a cation exchange membrane (as diaphragm) it is necessary to stop the operation of the diaphragm type electrolytic cell because of an exchange of the diaphragm due to durability of the porous diaphragm and the cation exchange membrane, or accidental trouble. The electrolysis can be continued without stopping the operation of the entire electrolyzing plant, but only by connecting the jumping switch C to both terminals of an electric circuit of an electrolytic cell for dominant state (electrolytic cell A.sub.2 in the drawings) as shown in FIGS. 1, 2 and 3.
When the operation of the electrolytic cell is temporarily stopped by the jumping switch for the electrolytic cell, and the operation is restarted after the exchange of the diaphragm in an electrolytic cell equipped with the electrode coated with a surface layer made of palladium oxide as the anode, it has been found that the chlorine overvoltage of the anode and the cell voltage rise higher than the voltages before the stop of the operation of the electrolytic cell and the economical operation of the electrolytic cell can not continue in a practical operation for ten to several hundreds hours after the restart of the operation. The electrodes made of such platinum group metal oxide should be usually used for 3 to 5 years whereas the durability of the diaphragm is usually only for 1 to 2 years. Therefore, such problem is a fatal disadvantage for the electrode used in the industrial operation.
The jumping switch is also used as a switch which bypasses the electrical current around each incapaciated cell to the two adjacent cells in a plant, thus allowing steady operation of the cell circuit without any interruptions due to the incapacity of a cell.